appliance: Make appliance building thread-safe (RHBZ#790721).

Appliance building can be called from multiple processes, but this is
only safe if each process holds a lock on the 'checksum' file.
However threads within a process are not excluded by a file lock, and
so this strategy completely failed for a multithreaded program calling
guestfs_launch in parallel.

Since it makes no sense for threads in a single program to race each
other to try to create the appliance, add a lock around appliance
building.

This serialises building the appliance, but the rest of guestfs_launch
(eg. starting up qemu) can run in parallel.

0 files changed, 0 insertions, 0 deletions